Despite what you may think, your heart rate doesn’t change depending on how you fit are, rather the more fit you become, the more work you can do at your maximum heart rate and this - along with your recovery rate - is what correlates with your level of fitness.
Why does your heart rate increase when you exercise?
When you start exercising your muscles use up more energy and to make more energy your muscles need extra oxygen that is pumped to them by the heart. The speed your heart is pumping blood at reflects your heart rate number. So if you're on a treadmill and increase the speed you're running at, your muscles will need more oxygen and your heart will have to work that much harder - pumping more blood around the body and therefore increasing your heart rate.
What is a heart rate monitor?
A basic heart rate monitor usually consists of a transmitter and a receiver and will measure your heart rate from as soon as you turn it on to when it gets turned off, so you can finish a workout with data on your heart rate from the entire session.
The transmitter essentially detects electrical activity which is then relayed by the graphic display of the watch that is looped around your wrist. Like the majority of electrical devices on the market there's a range of sophistication when it comes to heart rate monitors, from ones that simply measure your heart rate through to those that are programmed to bleep if you've gone below or above your preferred heart rate.
How should I know what heart rate to exercise at?
This is where some heart rate terminology comes in. Your maximum heart rate is basically what your heart rate reads when you have pushed yourself to extremes during a workout - the maximum number of beats your heart can take per minute. Serious athletes have methods of finding this number out but ordinarily, you should be careful trying to do this, as pushing yourself too hard during exercise will cause your body harm. A rough guide to measure your maximum heart rate is to subtract your age from the number 220.1
Your training heart rate is what you should be reaching - or lowering yourself down to - in order to improve your fitness. By using a heart rate monitor you can establish a rhythm of exercising that sticks to this particular heart rate. The Mayo Clinic Healthy Living Program recommends a target heart rate of 65 percent to 75 percent of your maximum heart rate for moderate-intensity exercise.2
Keeping an eye on your heart rate and sticking within these targets will help you improve your fitness goals without pushing your body too hard.
How does this help with my fitness?
Just as your body gets stronger with exercise, so does your heart. Whether you're lifting weights at the gym, cycling, running or sweating your way through an indoor spinning class, your heart will be working overtime to provide all your other muscles with the oxygen they need to keep on functioning. And by exercising for longer and longer at your optimum heart rate, your heart will get used to pumping blood (oxygen makes it to your muscles from the lungs, via the bloodstream) faster and faster around your bloodstream, making your heart resultantly stronger.
This is why - when you're resting - a lower heart rate is actually better than a higher one. A weaker heart pumps less blood around your body than a stronger one.
Knowing what your heart rate is doing allows you to work out which of your exercise routines are benefiting you most and whether you're pushing your body to hard or not hard enough to achieve your fitness goals.